Recepient address gets replaced with "@"@internalFQDNofMailserver

when creating and sending an email from deskapp or webapp with a recepient address that is available under the users contacts the recepient address gets replaced like : donald.duck@eh.com to “@”@mailserver.domain.intranet in outlook

if the mail is created and sent with outlook everithing is ok and also the right recepient address is shown in webappp/deskapp

also if i type in a new recepient which is not saved in the contacts it works as expected

I only have Kopano (8.3.2) Installations on Univention UCS (4.1-4, 4.2-1) - but there on all installations its the same behaviour (z-push 2.3.7, 2.3.8 and KOE 1.4 ,1.5 tested - but was also on earlier versions - but i think it is not related to KOE but to z-push)